By combining a conical type field emission gun and an auto gain controlled noise cancelling system, we have developed three types of digital FESEMs equipped with a digital imaging processor and three different sized specimen chambers each with a stage.
It is known that when a cold field emission gun (C-FEG) with a W (310) single crystal tip is used at a vacuum pressure of 10-10 torr, the emission current constantly fluctuates by 5 to 10% due to the adsorbed gas, etc. on the tip surface.
Since the probe current in an FESEM equipped with this C-FEG fluctuates to the same extent, the noise caused by emission current fluctuation (emission noise) appears on secondary electron images (SEI) and backscattered electron images (BEI).
In order to eliminate emission noise, an aperture (noise cancelling aperture or N/C aperture) installed under the C-FEG detects emission current fluctuation and inputs it to the differential amplifier of the video amplifier system for SEI or BEI on conventional FESEMs.
With FESEMs, however, when the accelerating voltage is change in the range from 0.5 to 30 kV, the virtual source of an FEG using Butler type electrodes moves several tens of centimeters on the optical axis. Moreover, the probe current is changed from 10-13 to 10-10 A by changing the excitation current of the condenser lens. For these two reasons, there have been adopted such methods as installing an N/C aperture in two positions (under the C-FEG and at the objective lens aperture position) and controlling the amplifier value of the noise cancelling system and condenser lens excitation by ROM data.